Problem:
If the shell command passed to the filtered bang command isn't found,
the error isn't redirected to the temp.out file when shell is set to
powershell.

Solution: Use anonymous function with Invoke-Command operator (&).

Problem:
`Start-Process` requires the command to be split into the shell
command and its arguments. Previously it was done by parsing, which
didn't handle cases such as
  - commands with escaped space in their filepath
  - quoted commands with space in their filepath

Solution:
Use
  - `pwsh -Command` instead of `Start-Process`
  - `Get-Content` instead of `-RedirectStandardInput`
  - `Out-File` instead of `-RedirectStandardOutput`

Problem:
Since 0b9664f5240be4d9e9d6882fcd398970fd3a9532 powershell filtered
:[range]! commands with args causes error:
"Start-Process: A positional parameter cannot be found that accepts argument ..."

Solution:
Pass args to Start-Process via `-ArgumentList`.
closes #19250

Problem:
The "calls executable in $PATH" is skipped on some CI jobs because
"python" is not found.

Solution:
Use "python3" instead.

Also:
- Add a describe('shell :!') section to system_spec.
- Make the test for #16271 work on systems without powershell.

Problem:
Nvim fails to create tempfile "…/nvim6UJx04/7" when 'shell' is set to
pwsh (PowerShell Core). This breaks filtered shell commands ":{range}!".
With shell set to cmd, it works.

Solution:
PowerShell doesn't use "<" for stdin redirection. Instead, use
"-RedirectStandardInput".

Closes #15913

Co-authored-by: Elias Alves Moura <eliamoura.alves@gmail.com>
Co-authored-by: venkatesh <shariharanvenkatesh@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Vikas Raj <24727447+numToStr@users.noreply.github.com>
Co-authored-by: Steve Vermeulen <sfvermeulen@gmail.com>
Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Co-authored-by: rwxd <rwxd@pm.me>
Co-authored-by: casswedson <58050969+casswedson@users.noreply.github.com>

Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua